1. Field of the Invention
The present invention relates to a fixing device that fixes an image, which has been electrostatically transferred onto a print medium, onto the print medium by heat and pressure, an image forming apparatus, and a fixing method.
2. Description of the Related Art
Electrophotography has been extensively used in printers, particularly in printers for high-volume output at high speed. Some type of the electrophotographic printers include a fixing unit that applies oil onto a heating roller and wipes off remaining toner on the heating roller to prevent transfer of residual toner. Such an electrophotographic printer is typically configured such that a fixed amount of oil is constantly supplied to an oiling web, such as felt, from an oil tank by using a pump or the like to apply the oil onto the heating roller.
Examples of such an image forming apparatus that utilizes a fixing unit include electrophotographic image forming apparatuses that form images at high speed on a web (imaging web), which is an elongated recording medium or a medium carrier sheet. One example of such an electrophotographic image forming apparatus is disclosed in Japanese Patent Application Laid-open No. 2008-158201.
Japanese Patent Application Laid-open No. 2008-209585 discloses a fixing unit that controls amount of oil to be supplied to an oiling web from an oil tank by changing rotation speed of an oiling roller, thereby preventing excessive oil on the oiling web.
Japanese Patent Application Laid-open No. S63-101882 discloses a fixing unit that includes: an endless oiling web that applies oil onto a heating roller of the fixing unit to clean the heating roller; and a cleaning mechanism that is compact but capable of accommodating a great length of the oiling web by folding the oiling web in accordion fold. This technique allows a fixed amount of oil to be supplied to the heating roller 11 constantly. Unfortunately, this technique is disadvantageous in that, because the oiling web is in constant contact with the heating roller and continuously supplying oil to the heating roller, amount of oil supplied to the surface of the heating roller is uncontrollable.
Japanese Patent Application Laid-open No. 2000-321914 discloses a fixing unit, in which an elastic member presses an oiling web against a fixing roller via a pressing roller at a constant pressure, and an elastic member presses a cleaning blade formed from heat-resistant resin or metal against the fixing roller at a constant pressure in an upstream side of the position of pressing the oiling web, in order to prevent dust from sticking to the fixing roller.
Japanese Patent Application Laid-open No. 2000-214716 discloses a technique for adjusting various parameters that include amount of oil to be applied, a feed speed of a cleaning web, pressure of the cleaning web, pressure of a separation claw, fixing temperature, and a conveying speed of recording paper in order to reduce jam of the recording paper in a fixing device.
Description will be made with reference to FIG. 9A and FIG. 9B. Generally, oil 18 applied to a heating roller 11 of a fixing unit is continuously consumed on the heating roller 11 by an imaging web 6 during the imaging web 6 is conveyed. Before conveyance of the web, the oil 18 is not consumed by the imaging web 6 because the imaging web 6 is out of contact with the heating roller 11 as illustrated in FIG. 9A (S301). Accordingly, before conveyance of the web, a large amount of oil 18 is applied onto a surface of the heating roller 11 using an oiling web 10 compared to during the conveyance of the imaging web 6. If printing is performed using an imaging web to which sticker paper 6p is glued, then immediately after conveyance of the imaging web is started, a large amount of oil 18 on the heating roller 11 is transferred onto a surface of the sticker paper 6p as illustrated in FIG. 9B (S302), thereby causing a reduction of pressure of the sticker paper 6p against the imaging web 6.
If a small amount of oil 18 is supplied to the oiling web 10, appropriate oil supply can be attained before the conveyance of the imaging web as illustrated in FIG. 10A (S401). During the printing, however, a tiny amount of oil 18 is applied to the sticker paper 6p as illustrated in FIG. 10B (S402), which causes poor releasability between a toner image 5 and the heating roller 11. As a result, the toner image 5 undesirably adheres to the surface of the heating roller 11. Once the oil 18 is supplied to the oiling web 10, it is substantially difficult to greatly reduce the amount of the oil 18 only while the conveyance of the imaging web is stopped. Therefore, it is difficult to reduce the oil supply to the surface of the heating roller 11 only while the web conveyance is stopped.